1. Field of the Invention
The present invention generally relates to a fluid-filled type vibration-damping device for producing vibration damping action based on flow action of a non-compressible fluid sealed inside, and more particularly to a fluid-filled type vibration-damping device suitable for use as an automotive engine mount, for example.
2. Description of the Related Art
As one type vibration-damping connector or vibration-damping support for installation between members making up a vibration transmission system, there has previously been proposed a fluid-filled type vibration-damping device of construction having a first mounting member and a second mounting member spaced apart from one another and connected by a main rubber elastic body; a pressure-receiving chamber a portion of whose wall is constituted by the main rubber elastic body; and an equilibrium chamber a portion of whose wall is constituted by a flexible film; as well as having an orifice passage interconnecting the two pressure-receiving chamber and the equilibrium chamber.
In such fluid-filled type vibration-damping devices, on the basis of resonance action of the fluid induced to flow through the orifice passage, it is possible to derive vibration damping action not readily achieved with the main rubber elastic body alone. Consequently, the fluid-filled type vibration-damping devices have been provided as an automotive engine mount and the like. Among these, there has been proposed a fluid-filled type vibration-damping device that, for reasons relating to vehicle layout, has the pressure receiving chamber formed to one side of the main rubber elastic body, the equilibrium chamber formed to the other side, and with the device installed in the automobile, the pressure-receiving chamber is positioned below the main rubber elastic body while the equilibrium chamber is positioned above the equilibrium chamber.
In order to achieve effective vibration damping action in a fluid-filled type vibration-damping device, it is important that flow of fluid through the orifice passage occur smoothly and stably.
The assignee proposed in the prior U.S. patent application Ser. No. 10/188,243 (U.S. Publication No. 2003-0001322 A1), a construction employing a generally frustoconical main rubber elastic body having in proximity to the large-diameter end of an upper tapered face thereof an opening which leads to the equilibrium chamber side of the orifice passage, and having a guide groove extending in a straight line from this opening towards the small-diameter end of the upper tapered face of the main rubber elastic body. By means of furnishing this guide groove, it is possible to stabilize the condition of fluid flow between the orifice passage and the equilibrium chamber, which is particular prone to unsteady fluid flow conditions or pressure distributions caused by deformation of the flexible film; and on the basis of this stabilization to improve and stabilize vibration damping action based on resonance or other such flow action of fluid induced to flow through the orifice passage.
However, as a result of further investigation, the inventors have found that, depending on the condition of installation of the vibration damping device, the construction taught in the aforementioned prior U.S. patent application may have some unresolved problems.
Specifically, when relatively low pressure is created in the pressure-receiving chamber in association with input of vibration, thereby causing fluid to flow from the equilibrium chamber into the pressure-receiving chamber through the orifice passage, the guide groove (which as described above is formed so as to open onto the surface of the main rubber elastic body) becomes covered up by the flexible film, posing the risk that the orifice passage will become closed off. In particular, in a fluid-filled type vibration-damping device of the design described previously having a pressure-receiving chamber and an equilibrium chamber formed above and below the main rubber elastic body, the action of gravity on the fluid sealed in the equilibrium chamber and on the flexible film also acts in the direction such that the opening of the guide groove, i.e. the opening of the orifice passage, is closed off by the flexible film. Additionally, by means of fluid flow guiding action of the guide groove, large pressure fluctuations along the guide groove (which extends in a straight line from the opening of the orifice passage into the equilibrium chamber) tend to be exerted on the equilibrium chamber, and thus there is a risk that localized large pressure fluctuations will occur in the area of the guide groove, whereby the flexible film tends to be easily pulled against the opening of the guide groove.
In recent years, there has been a trend towards making the amount of sealed fluid relatively small for the purpose of reducing the weight of vibration damping devices. Under this trend, the problem of the orifice passage becoming closed off will tend to become more imminent.
Also, in recent years, there have been proposed fluid-filled vibration-damping devices of dynamic type, furnished with an electromagnetic actuator so as to be able to actively control pressure fluctuations in the pressure-receiving chamber. However, with such dynamic type fluid-filled vibration-damping devices, since the intention is to actively produce relative pressure fluctuations of the pressure-receiving chamber and the equilibrium chamber and increase the flow of fluid through the orifice passage, the problem of the orifice passage becoming blocked off due to the guide groove being covered by the flexible film as described above will tend to be more likely to occur.
If the guide groove becomes covered by the flexible film in this way, the fluid flow guiding action of the guide groove, as well as the vibration damping action per se, which is based on the flow action of the fluid induced to flow through the orifice passage, will unavoidably experience an appreciable drop. A further problem is that repeated contact of the flexible film against the corner of the opening may cause reduced durability of the flexible film.
To address such problems, it may be contemplated, for example, to employ a special multidirectional branched configuration for the guide groove so that localized pressure fluctuations are dispersed. However, not only would a guide groove of such complicated shape be difficult to produce, but the flow of fluid along the guide groove would no longer be smooth. This would have the result of hampering the basic purpose of the guide groove, namely, to increase the flow of fluid through the orifice passage and to stabilize fluid flow.